The present invention relates generally to differential amplifier circuits, and in particular to linear differential transconductance amplifier circuits which are compensated in some manner to reduce error.
Simple differential transconductance amplifiers produce an undesirable error output current if a large differential voltage input is present or if thermal distortion is present due to transistor self heating. Several transconductance amplifier circuits have been disclosed which use a correction signal to reduce this error current by using a separate error amplifier to linearize the output of a main signal amplifier. In U.S. Pat. Nos. 4,146,844, 4,322,688, amplifier circuits include a main differential amplifier, a common base stage, and an error amplifier whose inputs are connected to the emitters of the common base stage and whose outputs are connected to the collectors of the common base stage. In U.S. Pat. No. 4,390,848 an amplifier circuit includes a main differential amplifier having an emitter resistor, and an error amplifier whose inputs are connected across the emitter resistor and whose outputs are connected to the outputs of the main differential amplifier.
These prior art circuits, while capable of reducing unwanted error current, do so at the expense of additional power consumption. For the above configurations to work, the error amplifier itself must be perfectly linear. That is, additional errors may not be introduced by the error amplifier itself. The most common technique for ensuring the linearity of the error amplifier is by maintaining sufficient emitter bias current. This bias current, however, is not put to any other useful purpose in the circuit.
What is desired is a linearized differential amplifier in which the error amplifier bias current may be reused by the main differential amplifier to produce greater linearity with less power consumption.